The performance of turbine engines for both power generation and aircraft propulsion has made dramatic advances in recent decades due to advances in materials technology. In turbine engines for aircraft, a high output to weight ratio is desired. Engine efficiency increases with increasing temperatures in the combustion section. The temperature limiting factor in this application is the availability of materials of construction for turbine blades. Turbine blades are commonly made of nickel-based superalloys. Advances in casting techniques--such as investment casting, which permitted production of blades with complex internal passages for air cooling; casting of nickel superalloys with directional solidification in the investment casting process which ultimately led to single crystal superalloy blades; and finally the development of surface coatings to protect against oxidation and corrosion--have dramatically increased the temperature performance of such blades. However, metals technology is approaching the upper temperature limit, and new materials of construction are needed to provide further advances. Oxide crystals offer themselves as material of construction for turbine blades because of high theoretical strength and oxidation resistance. However, lack of mechanical durability and strength in actual application has heretofore prevented their use in this demanding application. We have investigated oxide crystals for potential use in this application and have found that those of one particular class, namely rare earth aluminum garnets, have the necessary strength and creep resistance, provided they can be protected against failure by brittle fracture. We have found that such protection can be provided for service at surprisingly high temperatures by careful surface polishing followed by deposition of an epitaxial (single crystal) garnet layer which puts the surface of the blade under significant compression.
Compressive surface layers are employed widely to improve the low temperature strength and impact resistance of brittle solids and objects. A common example is "tempered" glass for automotive and architectural applications. Surface compression in glass and metallic materials has been achieved by a variety of methods including heat treatment, shot peening and ion exchange. The surface stresses produced by these methods, however, generally relax upon exposure of the solid to temperatures in the neighborhood of 0.5 or less times the absolute melting point.
Compressive epitaxially deposited layers have previously been provided on single crystal laser media, including on single crystal yttrium aluminum garnet laser rods for improvement of low temperature strength and durability (U.S. Statutory Invention Registration H557 by Morris et al. for "Epitaxial Strengthening of Crystals"; Marion et al., Compressive epitactic layers on single-crystal components for improved mechanical durability and strength, J. Appl. Phys. 62, 2065-2069 (1987)).